Each time it is necessary to maintain a light polarization state stable over a period of time, in spite of external constraints (changes in temperature, pressure, . . . ), the conventional low loss monomode optical fiber for optical telecommunications must be replaced by a special fiber which manages to maintain polarization. Because of residual imperfections (slight ellipticity, anisotropic stresses), conventional "circularly-symmetrical" monomode fibers are capable, in practice, of conveying two orthogonal polarized propagation modes, and because of mode coupling effects the polarization state at the outlet from the fiber is unstable.
Linear polarization maintaining fibers have linear birefringence introduced by removing the degeneracy of the two linear polarization states of the fundamental mode, either by shape birefringence (elliptical core), or by stress birefringence (anisotropic stress around the core), or else by a combination of both. Such fibers are advantageous for networks, long distance links, and coherent detection optical telecommunication systems, and also for some optical fiber sensor applications, e.g. hydrophones.
Various methods of making linear polarization maintaining fibers are known, and are described in reference [1] "Polarization maintaining fibers and their applications" by J. Noda et al. published in the Journal of Lightwave Technology, Vol. LT-4, No. 8, pp. 1071-1089, 1986. "Bow-tie" fibers are provided by MCVD deposition of the cladding and the core on a tube including diametrically opposite circular sectors which are doped with boron.
In reference [2] "Substrate tube lithography for optical fibers" by R. H. Stolen et al. published in Electronics Letters, Vol. 18, No. 18, pp. 764-765, 1982, these sectors are made by selective chemical etching. This technique is difficult: it consists in using a solution of hydrofluoric acid to etch sectors of boron oxide doped silica that have not been protected by deposition of a photosensitive resin. There are numerous steps: deposition, UV exposure, development, etching, etc.
In reference [3] "Fabrication of polarization-maintaining fibers" by R. D. Birch et al. in Electronics Letters, Vol. 18, No. 24, pp. 1036-1038, 1982, these sectors are made by selective thermal etching. Etching is performed in this case by hydrogen fluoride gas acting on portions that have been heated by means of a blow lamp.
In both preceding cases, subsequent deposition of the optical cladding and of the core is performed on an asymmetrical structure which leads, after collapsing, to a non-circular core, thereby increasing losses on coupling to a standard line fiber having a circular core. In the second case, it is possible to reduce this effect by an additional stage of localized redeposition, as described in British patent application GB-A-2 180 232. This makes the method even more complicated and even so the result is far from perfect.
In reference [4] "Polarization-maintaining fiber" by Y. Sasaki et al., published in Electronics Letters, Vol. 19, pp. 792-794, 1983, a "pin-in-jacket" or "panda" method is implemented requiring the envelope to be pierced, which is a difficult operation to implement over a long length without running the risk of breaking the preform.
In all of the methods methods above, zones of boron oxide doped silica are inserted on either side of the core. In order to obtain high birefringence, it is necessary to insert zones in the preform which give rise to high levels of stress, and very often this leads to the preforms cracking or even bursting.
Finally, European patent application EP-A-0 145 031 describes a method in which the following are disposed around a basic preform inside a tube of silica cladding glass, firstly two cylinders of silica doped at least with boron oxide, and secondly two cylinders of silica doped with titanium oxide. The empty spaces inside the tube are provided with rods of non-doped silica. It is extremely difficult to maintain all these items in their proper positions relative to one another. It is observed that fibers produced by this method have core deformations which is a handicap when coupling to genuinely circular fibers, or else they have external deformations which complicates subsequent handling of the fiber.
The object of the present invention is therefore to avoid the above-mentioned drawbacks and to provide in a simpler manner a high birefringence polarization-maintaining fiber which is easy to couple or connect to network fibers.